From past to now, rear-projection type projection television sets have been known which enlarged the image light from an image projection device and projected the light onto a screen from the rear, to enable viewing of the image. As the image projection device, a CRT type device can be used, and in recent years liquid crystal displays (LCDs), digital micromirror devices (DMDs), and other micro-displays have also been used.
A rear-projection type screen has a configuration in which two sheets, which are a fresnel lens and a lenticular lens sheet, are stacked and combined. Further, a front sheet may be provided on the viewing side as well. If a gap exists between these two lens sheets, the image may be defocused, or other problems may occur, and so it is required to pile the two sheets in close contact without such gaps. Further, if the stacked lens sheets are not planar overall, there is the problem that warping of the image or other problems occur, and so an overall planar shape must be maintained. Also, in a lens sheet for use in the screen, even the sheet need not be rigorously planar, but may be somewhat convex on the light source side. This is because even if the stacked lens sheets are convex on the viewing side, upon pressing the screen with a finger, the screen is depressed, but when pressing a depression, the depression remains without changing.
Thus, technology is sought to bring the stacked lens sheets into close contact, and to retain an overall planar shape for the two lens sheets, or a somewhat convex shape on the light source side. For example, Japanese Patent Laid-open No. 4-67134 discloses a method in which a fresnel sheet is bowed in advance to be convex on the emission-face side and a lenticular sheet is bowed in advance to be concave on the incidence-face side, and the two are combined and placed in close contact. And, Japanese Patent Laid-open No. 2-072341 discloses a method in which three sheets are all formed in a convex bowed shape in the same direction, and are stacked and placed in close contact.
In general in a rear-projection type screen, multilayer sheets formed by coextrusion molding and lamination are used. In such multilayer sheets, there is the problem that bimetal warping occurs due to differences in the linear expansion rates between layers or due to differences in moisture absorption rates between layers. To address such problems, in Japanese Patent Laid-open No. 11-072848, a method is proposed in which a multilayer sheet is used having a structure such that absorptivity is smaller on the viewing side than on the image side, so that upon moisture absorption, warping toward the image side occurs.
In Japanese Patent Laid-open No. 11-072848, an absorptivity difference of 0.1% or greater is preferable, and no problems are thought to occur no matter how great the difference. Here, in actuality there is the problem that, if the difference is too great, extreme warping occurs. Further, as examples of materials having preferred absorptivities, “acrylics, polycarbonates, styrenes” and similar are disclosed; but there is no mention of the Young's modulus, coefficient of extension, or layer thicknesses, which are important in relation to warping changes, so that specificity is lacking.
Further, in Japanese Patent Laid-open No. 2002-04563, technology is disclosed in which the linear expansion rate difference Δα between a laminated lenticular lens sheet and fresnel lens sheet is made 5.5×10−5 (/° C.) or less. In this technology, material having a linear expansion rate of 3×10−5 (/° C.) or less is further laminated on the uppermost surface. By this means, bimetal warping changes are suppressed, and the direction of occurrence of warping is controlled so as to result in convexity on the image light source side. In this Japanese Patent Laid-open No. 2002-04563 also, there is no mention of the Young's modulus, which is important in relation to warping changes, and specificity is lacking.
In this way, technology to reduce warping changes in sheets with respect to temperature changes have been disclosed in the past. On the other hand, with the object of obtaining multifunctionality in screens, in recent years there have been numerous sheets with multilayer configurations using different materials. In a multilayer sheet, if there is a difference in linear expansion coefficients, sheet warping changes similarly to bimetals with temperature changes.
A diffuser plate to prevent brightness unevenness, and a lens sheet or similar to condense light which has passed through the diffuser plate, and similar are mounted to the backlight provided in the rear portion of the liquid crystal display panel. The lens sheet is formed by layering a base material sheet layer and lens layers of different materials, and is formed by layering a plurality of diffuser sheets. In a liquid crystal display apparatus, in addition to changes in the ambient temperature, heat emitted from the light source positioned therewithin and other heat may be generated, so that there is the problem of the occurrence of sheet warping of the lens sheet of a plurality of layers.
In order to suppress sheet warping, methods such as reducing the differences between the linear expansion coefficients of the different layers, or of making one of the layers extremely rigid, or similar are conceivable. In Japanese Patent Laid-open No. 2006-208930, a configuration is disclosed in which a layered sheet is formed from different materials or formed by laminating a plurality of light diffuser sheets. However, due to the multifunctionality of the sheet itself, problems with sheet manufacture, and similar, it is not necessarily possible to set appropriate values of the thermal expansion coefficients, Young's modulus, and thickness for each of the layers, and in some cases conventional technology cannot be applied.
Further, in liquid crystal display apparatuses in particular, when warping occurs in a lens sheet which is to be incorporated in a backlight, there is the possibility of inducing (1) partial damage to the lens sheet, due to contact of the lens sheet with the liquid crystal display panel or light source, and (2) brightness unevenness, arising from changes in the view angle created by the lens sheet.
In Japanese Patent Laid-open No. 2007-109608, technology is disclosed in which flexing-prevention pins are provided on the light source side such that, when the sheet is warped into a convex shape on the light source side, the effect of the warping is alleviated. However, when the amount of warping is large, contact between the lens sheet and the flexing-prevention pins may induce partial damage in the lens sheet. Also, using the technology of Japanese Patent Laid-open No. 2007-109608, it is difficult to accommodate cases in which the lens sheet is warped in a convex shape on the liquid crystal display panel side.